Dominic Burford - Professional Profile

namespace CoreLibrary
{
    /// <summary>
    /// We use this interface for contructor injection in our unit tests. 
    /// We are essentially mocking the DateTime.Now property for unit testing.
    /// </summary>
    public interface IDateTime
    {
        DateTime Now { get; set; }
    }

    /// <summary>
    /// Library functions used by the application
    /// </summary>
    public class ReportLibrary
    {
        private readonly IDateTime _datetime;

        private class ActualDateTime : IDateTime
        {
            public DateTime Now { get; set; }
        }

        //If no reference to IDateTime is passed to the constructor then default to 
        //using the actual DateTime.Now property
        public ReportLibrary()
        {
            this._datetime = new ActualDateTime { Now = DateTime.Now};
        }

        //We will inject a reference to IDateTime from our unit tests with the required value for DateTime.Now specified
        public ReportLibrary(IDateTime datetime)
        {
            this._datetime = datetime;
        }
        public int MyFunction()
        {
          int result;

          //The original line of code below
          //DateTime today = DateTime.Now;

          //This is the updated line of code
          DateTime today = this._datetime.Now;

          //rest of the calculation goes here
          return result;
        }
    }
}

ReportLibrary reportLibrary = new ReportLibrary();
int result = reportLibrary.MyFunction();

[TestClass]
public class ReportLibraryTests
{
  //this goes at the top of our unit test class
  private class MockDateTime : IDateTime
  {
    public DateTime Now { get; set; }
  }
  //rest of the test methods go here
}

[TestMethod]
public void MyFunctionTests()
{
  IDateTime dateTime = new MockDateTime();
  //set the DateTime.Now property and inject this into the ReportLibrary constructor
  DateTime today = new DateTime(2016, 8, 3);
  dateTime.Now = today;
  ReportLibrary reportLibrary = new ReportLibrary(dateTime);
  int result = reportLibrary.MyFunction();
  Assert.AreEqual(999, result, "Invalid result for 'MyFunction'");
}